There are three methods for classifying powdered material, which are a sieving method, a dry classifying (air-classifying) method and a wet classifying method. The present invention relates to sieving and air classification.
Air classification that classifies particles by the agency of balance of inertial force produced by air currents with gravity can classify small particles of particle sizes on the order of 1 μm by aptly using centrifugal force or the like. However, air classification is not satisfactory in classification accuracy because small particles classified by the air classification contain large particles and large particles classified by air classification contain small particles.
Classification using a screen, namely sieving, is satisfactory in sieving accuracy (classification accuracy). However, a screen of a smaller mesh size is liable to be clogged up. Therefore, the lower limit of a particle size range of small particles that can be screened by sieving is considerably large.
Various air sieving apparatus designed to prevent clogging of a screen have been proposed.
Basically, the air sieving apparatus is a sealed structure formed by covering a screening case excluding a powder feed opening with a lid. An internal space in the screening case is divided into upper and lower spaces by a screen. Usually, particles are fed onto the upper surface of the screen, the particles are dispersed by some means, and air is sucked downward through the screen to screen the dispersed particles by the screen.
In an air sieving apparatus disclosed in Patent Document 1, particles clogging the meshes of a screen is removed by ejecting a gas from a slotted nozzle that rotates along the lower surface of the screen to prevent the screen from being clogged up.
In an air sieving apparatus proposed by the inventors of the present invention in Patent Document 2, an upper end opening of a screening case is so covered with a lid that gaps are formed between the upper end surface of the screening case and the lid, and particles fed onto the upper surface of a screen are dispersed by air currents sucked through the gaps into an upper space in the screening case. Air currents and the particles entrained by the air currents flowing along the upper surface of the screen perform a screen cleaning effect of scraping particles caught in the meshes of the screen off the screen to prevent the screen from being clogged.
Although the sieving method described in Patent Documents 1 and 2 use air currents and classify particles by the screen, the same do not exercise the effect of air classification that uses the balance of inertial force produced by air currents with gravity.
A classifying apparatus disclosed in Patent Document 3 uses both screening and air classification.
The classifying apparatus disclosed in Patent Document 3 sprays powder into an ascending jet of air in a lower casing through a powder feed opening toward a classifying screen disposed above the powder feed opening. Fine particles that have passed through the classifying screen are collected in a space over the classifying screen and coarse particles are collected in a space under the classifying screen. The classifying apparatus is provided with a rotary air brush above the classifying screen, the air brush having slits through which high-pressure air is supplied to prevent the classifying screen from being clogged.    Patent Document 1: JP 2002-186908 A    Patent Document 2: JP 2007-301490 A    Patent Document 3: JP H8-126848 A